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Spanish and English are two very different types of language, and this is especially true when it comes to pronunciation. In Spanish, we hit all the words as they come out in a sentence. However, in English, some words are more important than others. The words that are important are called strong words.The words that are not as important are called weak words. Sometimes words have a strong form and a weak form depending on whether we pronounce them in context or in isolation, fast or slowly.
In general, we could say that when a not-so-important word is pronounced fast and in context, the weak form appears because the word is linked to the word that precedes or follows it. A typical example is the auxiliary word have. If you pronounce it together with, for instance, the modal verb might, the slow pronunciation of MIGHT HAVE would be very different from the fast pronunciation, where linking between the two words happens (a phenomenon that in Linguistics is called liaison) and a weak form of have appears. 
To help you learn this difference, there is a free online talking dictionary of English pronunciation on howjsay.com. Type the words MIGHT HAVE in the search box and see what happens.